German Published Patent Application No. 199 03 439 describes a method and a device for operating an internal combustion engine in whose exhaust area an SCR (selective catalytic reduction) catalytic converter is provided which uses a reagent to reduce nitrogen oxides in the exhaust gas of an internal combustion engine to nitrogen. The reagent is preferably metered as a function of performance characteristics of the internal combustion engine such as the speed and the quantity of fuel injected. In addition, fuel is preferably metered based on operating parameters of the exhaust gas, such as the exhaust gas temperature or the operating temperature of the SCR catalytic converter.
For example, the reducing agent ammonia, which may be obtained from a urea-water solution, is used as the reagent. The reagent or precursors of the reagent must be metered carefully into the exhaust area. If the amount metered is too low, this results in nitrogen oxides no longer being completely decreased in the SCR catalytic converter. If the metering is too high, this results in leakage of reagent, which may result in unnecessarily high reagent consumption as well as an unpleasant odor problem, depending on the nature of the reagent. In addition, another factor to be taken into account is that ammonia is toxic.
German Published Patent Application No. 197 39 848 describes a procedure using which raw NOx emissions of an internal combustion engine may be calculated at least approximately from known performance characteristics of the engine. The starting point is an engine characteristics map, which is based on the speed and torque of the engine. In addition, corrections may also be provided, e.g., as a function of the lambda value.
German Patent Application No. 10 2004 031 624 (not a prior publication) describes a method for operating an SCR catalytic converter used for purifying the exhaust gas of an internal combustion engine; with this method, control or regulation of the reagent filling level in the SCR catalytic converter is provided at a predefined setpoint storage volume. The targeted specification of the setpoint storage volume ensures that an adequate quantity of reagent for the most thorough possible elimination of raw NOx emissions produced by the internal combustion engine is available in non-steady states of the engine while avoiding reagent leakage.
The reagent filling level of the SCR catalytic converter is determined on the basis of a catalytic converter model that takes into account the NOx mass flow entering the SCR catalytic converter, the NOx mass flow leaving the SCR catalytic converter, the catalytic converter temperature and, if necessary, the reagent leakage. The maximum possible reagent filling level of the SCR catalytic converter depends in particular on the operating temperature of the SCR catalytic converter. The maximum possible reagent filling level is highest at low operating temperatures and drops to lower values with increasing operating temperatures. The efficiency of the SCR catalytic converter depends on the reagent filling level and the catalytic activity, which is also low at low operating temperatures, passing through a maximum with an increase in operating temperature and then dropping again with a further increase in operating temperature.